1. Field of the Invention
This invention relates to a plug for manufacturing a seamless steel pipe.
2. Description of the Prior Art
Various kinds of plugs are used in drilling processes rolling processes (an elongator, a plug-mill, etc.) and polishing processes of manufacturing a seamless steel pipe. Since the surface of the plugs is used under a high pressure and a high shearing force at a high temperature, abrasion, melting loss, seize or the like often occurred. As a result, the life of the plug was shortened, and the quality of the internal surface was adversely affected. Thus, some inventions were provided in order to improve the life of the plug.
Seamless steel pipes are generally made of low alloy steel or high alloy steel. The low alloy steel is usually drilled with a plug having an oxide layer on the surface thereof made of low alloy steel of 0.3% C-3% Cr-1% Ni. For example, a heat treatment for forming the oxide layer on the surface of the plug is disclosed in Japanese Patent KOKAI NO. 58-19363, and a thermal spraying process of a powder mainly composed of iron oxide for forming the oxide layer is disclosed in Japanese Patent KOKAI No. 59-13924. According to the disclosure of the above patents, the plug manufactured by the above-mentioned processes could drill 500 to 1500 times a billet made of low alloy steel containing up to 2.25 wt. % of chromium to produce hollow pieces 4 to 8 meters in length, and therfore, the life of plug was elongated.
However, when a billet was made of high alloy steel such as Cr steel containing more than 13 wt. % of chromium or austenite stainless steel, the plug was seized remarkably due to a great strength at a high temperature of high alloy steel and due to the shortage of an iron oxide supplied from the billet to the surface of the plug prevented by the chromium oxide produced on the surface of the billet. As a result, a melting loss come to be remarkable as shown in FIG. 6. The plug accordingly could be used about five times at the longest, and occasionally, the plug could be used only once.
Various inventions were recently proposed to extened the life of the plug for drilling high alloy steels. For example, a combination of thermal spraying and hot isostatic pressing (HIP) process is disclosed in Japanese Patent KOKAI No. 61-286077. This process is used for coating with ZrO.sub.2 for the purpose of thermal insulation of an engine room. The plug was manufactured through the thermal spraying of a nickel thermal spraying material (Ni-Cr-Al-Y) powder to the surface of the core made of carbon steel (S45C) to a middle layer, and the thermal spraying of molybdenum powder to form a surface layer, followed by the HIP treatment. According to this reference, the effect of the middle layer is remarkable, and particularly, it relaxes the thermal stress of a junction layer accompanied with a rapid change of temperature at the surface of the core. Thermal shock is improved in comparison with the same of the plug having no middle layer.
To utilize canning HIP process is disclosed in Japanese Patent KOKAI No. 62-50009. This process is, for example, used for cladding a corrosion resistant material to an inner surface of a valve for an oil well. The plug is manufactured by coating a nickel powder to the surface of the core material made of low alloy steel (3% Cr-1% Ni steel) to form a middle layer, and then, coating molybdenum powder to form a surface layer, followed by HIP treatment. According to this reference, dense sintering of the coating layers are promoted, and at the same time, a metal coating layer having a high joining ability accompanied with a diffusion is formed on the interface between the surface layer and the core material.
Another canning HIP process is disclosed in Japanese Patent KOKAI No. 62-238011. The plug is manufactured using ceramics (Si.sub.3 N.sub.4, SiC) having a great strength at a high temperature and a high joining ability to a metal coating layer. According to this reference, a further excellent plug can be obtained by that dense sintering of the coating layers are promoted, and at the same time, a metal coating layer having a high joining ability accompanied with a diffusion is formed on the interface between the surface layer and the core material.
As mentioned above, the plugs disclosed in the above Japanese Patent KOKAI No. 61-286077, No. 62-50009 and No. 62-238001 are characterized in that the core material and the surface coating material are strongly joined at the joining interface accompanied with a diffusion. However, in the case of joining iron alloy to molybdenum, even if a nickel layer is provided therebetween, it is difficult to join them because of a great difference of thermal expansion coefficient between the iron alloy and the molybdenum. Moreover, when the ceramics such as Si.sub.3 N.sub.4 or SiC used have a greatly lower thermal expansion coefficient than molybdenum, a large tensile stress occurred in the molybdenum layer to be broken after the HIP treatment. As shown in the examples, even in the thermal shock test, the surface layer exfoliates on a heat shock of 15 to 30 times. In these plugs, the middle layer made of nickel alloy relaxes the thermal stress on the interface of the molybdenum coating layer and the core material. However, since the thermal stress between the coating layer and the core material is actually too great, the joining interface of the coating layer and the core material is already cracked at the time of the HIP treatment, so the coating layer exfoliates away by the heat shock test. Moreover, if crack or exfoliation occurs on the coating layer, it slides on the core material and is readily broken by a high pressure and a high shearing force at actually drilling a billet or rolling.